During the extrusion of a plastic, in particular of polyolefins, flow irregularities appear at the die exit when a critical shear rate is exceeded. Below this critical rate the extruded materials are smooth, whereas above it surface defects are observed. These defects, called “melt fracture”, take several forms. At a shear rate slightly above the critical rate, films obtained by extrusion-blowing lose their transparency and their gloss. At substantially higher rates, corresponding to higher productivity, inhomogeneities with smooth regions in a rough surface appear. These defects significantly reduce the optical and mechanical properties of the film. The same phenomena may be observed on extruded rods. When the surface of the rods loses its gloss, becoming dull and rough, it is often compared with an “orange peel”.
The prior art has already proposed the addition of processing aids.
Thus, in U.S. Pat. No. 3,334,157, the incorporation of polytetrafluoroethylene improves the optical properties of the polyethylene film.
According to U.S. Pat. No. 4,855,360, U.S. Pat. No. 5,587,429, U.S. Pat. No. 6,734,252 and U.S. Pat. No. 6,894,118, a fluoroelastomer in combination with a polyoxyalklene (polyoxyethylene glycol denoted also by PEG) is used to improve the conversion of hydrocarbon polymers. However, the fluoropolymer and the PEG are added as such, without any particular precaution, to the polyolefin in order to form the masterbatch. In these examples of the prior art, the fluoropolymer and the PEG are diluted in a polyolefin to form a masterbatch, and then the masterbatch is added to the polymer to be extruded.
U.S. Pat. No. 5,015,693, the PEG and the fluoropolymer may be mixed both in the melt state before being introduced into the material for extrusion. Mixing cannot be very intimate. This is because a fluoropolymer generally has a melting temperature of the order of 200-300° C. and because at this temperature the PEG exhibits a very low viscosity, making it poorly suited for intimate mixing with the fluorpolymer.
Organophosphates or organophosphites in combination with a fluoroelastomer have been described in U.S. Pat. No. 4,983,677 and U.S. Pat. No. 4,863,983, again to improve the conversion of hydrocarbon polymers.
It has been discovered that if the fluoropolymer and the PEG are intimately preblended before they are added to the polyolefin in order to form the masterbatch, the processing aid obtained is more effective. The term “more effective” is understood to mean in comparison with a masterbatch prepared by simple addition of the fluoropolymer and the PEG without intimate preblending before they are added to the polyolefin to form the masterbatch.